Many industrial gas streams are contaminated with sulfur compounds/species such as hydrogen sulphide (H2S), sulfur dioxide (SO2), carbonyl sulphide (COS), mercaptans (ie “RSH”, where R represents an alkane, alkene, or other carbon-containing groups of atoms), and/or carbon disulphide (CS2). For environmental or regulatory reasons it is typically mandated to remove or reduce the levels of such sulphur sulfur species in such industrial gas streams, such as in pipelines proximate to human habitation. It has been estimated that about forty percent or 2600 Tcf of the world's natural gas reserves are in the form of sour gas where H2S and CO2 compositions exceed 10% of the raw produced sour gas. Other gas streams also contain sulfur species: for examples, refinery gas, in situ combustion produced gas, and coal and petroleum gasifiers.
A number of prior art processes currently exist to remove specifically the H2S from gas streams.
For example, one approach is subject the gas stream to an acid gas removal unit, which removes substantial amounts of H2S and CO2 from the H2S containing stream. The off-gas from the acid-gas removal unit is mainly H2S and CO2. The sulfur from this off-gas stream is usually removed by the Claus reaction which produces saleable elemental sulfur. Specifically, the Claus process may be used for processing large volumes, and a liquid reduction-oxidation processes used for intermediate sour gas volumes, and H2S disposable scavengers for small volumes. However, these processes can be relatively expensive in capital and operating costs.
Below is a Review of Prior Art and Sulfur Species Chemical Reactions:
In the first step in the Claus process, about one third of the H2S present may be oxidized to SO2. In the second step, remaining H2S and SO2 are reacted in the presence of a Claus catalyst to form elemental sulfur in a series of Claus reactors according to Reaction 1:H2S+ 3/2O2→SO2+H2O  1a.2H2S+SO2→2H2O+3S Claus reaction  1b.
The Claus reaction is limited by thermodynamic equilibrium and only a portion of the sulfur can be produced. Therefore, multiple stages with sulfur condensation between the stages are used to increase the sulfur recovery factor. However, the effluent gas from a series of reactors in a Claus plant can contain varying amounts of different compounds including sulfur vapour, SO2, un-reacted H2S, COS, and CS2. Carbon disulphide is formed according to Reaction 2:CH4+4S→CS2+2H2S High temp. Claus furnace or combustion reaction  2.
Typically, this Claus plant effluent gas stream is burned with air to convert all sulfur-containing compounds in the stream to SO2 before discharge into the atmosphere. As environmental requirements are become stricter, the SO2 emission limit is being lowered, giving rise to the challenge of how to reduce or eliminate SO2 emissions.
Another process for the oxidation of H2S to elemental sulfur is described in U.S. Pat. No. 4,197,277 by the following H2S Oxidation Reactions 3a andH2S+0.5O2→S+H2O H2S Partial oxidation  3a.H2S+1.5O2→SO2+H2O H2S Complete oxidation  3b.
According to the '277 patent, the H2S-containing gas is passed with an oxygen-containing gas over a catalyst which comprises iron oxide and vanadium oxide as active materials and aluminum oxide as a support material. The catalyst described in the patent gives rise to at least a partial Claus equilibrium, so that SO2 formation cannot be prevented. Similarly, U.S. Pat. No. 5,352,422 describes a process for oxidizing the un-reacted H2S in the Claus tail gas to elemental sulfur. The patent describes a catalyst prepared by impregnation of an iron containing solution or an iron/chromium-containing solution into several carriers followed by calcination in air at 500° C.
U.S. Pat. No. 4,818,740 discloses a catalyst for the H2S oxidation to elemental sulfur, the use of which is said to prevent the reverse Claus reaction to a large extent. The catalyst comprises a support of which the surface exposed to the gaseous phase does not exhibit any alkaline properties under the reaction conditions, while a catalytically active material is applied to this surface. A modification of the method disclosed in '740 is disclosed in European patent 409,353. This patent relates to a catalyst for the selective oxidation of sulfur-containing compounds to elemental sulfur, comprising at least one catalytically active material and optionally a support. The described catalyst exhibits substantially no activity towards the reverse Claus reaction under the reaction conditions.
The direct oxidation of H2S to elemental sulfur is known to take place on a wide range of catalysts. However, many of the catalysts experience a rapid deactivation and fouling due to a high level of carbon and/or sulfur deposits and irreversible sulphation of the catalyst surface. Alumina-based catalysts are particularly susceptible in this regard. U.S. patent application 2005/0100504 relates to a process for selective oxidation of H2S to elemental sulfur in the presence of an inert liquid medium to moderate the reaction temperature and to remove the sulfur from the reaction zone. The inert medium used in this application could be water, produced liquid sulfur, or any other liquid that is not substantially consumed under the reaction conditions. The oxidation reaction was carried out at a temperature in the range of from 120-160° C. and a high pressure preferably in the range of from 60-120 bars (absolute) in order to maintain the supplied liquid in the liquid form during the oxidation process to enable it to remove the sulfur from the reaction zone. Nevertheless, carrying the H2S oxidation reaction at temperatures below the sulfur dew point and high pressures can force the produced sulfur to deposit inside the catalyst pore structures.
The gas streams from different chemical processes may contain a range of sulfur-containing compounds such as, H2S, SO2, COS, CS2 and RSH. Gases from combustion processes, such as in-situ combustion and coal or coke gasification may also contain CO, CO2 and H2. In the direct oxidation process, represented by Reaction 3a, oxygen is reacted with H2S over a catalyst to convert it to elemental sulfur. Because SO2 and COS are not altered in the catalytic direct oxidation process, a pretreatment process of the gas feed stream is conducted to convert sulfur-containing compounds to H2S so that a higher sulfur removal efficiency can be achieved. U.S. Pat. Nos. 4,552,746 and 4,857,297 relate to a process for the oxidation of H2S to elemental sulfur in the presence of oxidation catalyst and a feed gas stream comprising less than 10 vol % water. The feed stream is pretreated to convert the sulfur-containing compounds to H2S. The feed gas pretreatment could be accomplished in Reactions 4-7 by using, for example a dual hydrolysis/hydrogenation catalyst of cobalt or nickel/molybdenum on alumina to convert the undesirable components in the gas stream to H2S so that the stream would become amenable to direct oxidation:
HydrogenationRCH2SH+H2→H2S+RCH3 Mercaptan hydrogenation  4.SO2+3H2→H2S+2H2O Sulfur dioxide hydrogenation  5.CS2+4H2→2H2S+CH4 Carbon disulfide hydrogenation  6.COS+4H2→H2S+CH4+H2O Carbonyl sulphide hydrogenation  7.
The rapid deactivation of the H2S direct oxidation catalyst was addressed by carrying out the oxidation reaction at a temperature above the sulfur dew point at the reaction conditions. Canadian patent 2,318,734 relates to a process for passing a hydrogen sulphide-containing gas stream mixed with the oxygen-containing stream over a catalyst comprising niobium oxide and a promoter on a titanium dioxide carrier. The stability of the catalyst was investigated in the presence of water and carbon dioxide and at a temperature above the sulfur dew point to slow the deactivation of the oxidation catalyst due to the sulfur deposition. The H2S conversion to elemental sulfur was greater than 90% and sulfur selectivity was greater than 85%. Although the presence of CO2 in the feed gas stream increases the possibility of the COS formation (Reaction 11) during the H2S direct oxidation to elemental sulfur, the effluent gas was not analyzed for COS:H2S+CO2→COS+H2O  11.
Furthermore, the inventors of CA 2,318,734 and those of U.S. Pat. Nos. 4,552,746 and 4,857,297 apparently did not evaluate the performance of the disclosed H2S oxidation catalyst in the presence of a feed stream containing carbon monoxide, which undergoes side reactions during the H2S direct oxidation process to form COS:
CO ReactionsCO+S→COS  8.CO+H2S→COS+H2  9.3CO+SO2→COS+2CO2  10.
X-Ray diffraction analysis of the disclosed catalyst showed a homogenous mixture of the oxides of Nb and Ti. The presence of Nb oxides in the catalyst increases the number of the Lewis acid sites on the Ti surface (Jih-Mirn Jehng and Israel E. Wachs, Catalysis Today, 8, 1, 1990). As a result, the catalyst could became inactive for the COS and/or CS2 hydrolysis even at high temperature and in the presence of water (P. Grancher, C. Blanc, G. Guyot, M. Mathieu, J. Npugayrede, and J. Tessier, Inform. Chem., 199, 145, 1980).
Therefore, the concentration of COS and/or CS2 in the product gas effluent of the above patents is expected to be substantial, and a post-treatment process would be required to convert the produced COS to H2S, which in turn can be recycled to the H2S direct oxidation reactor to achieve a high sulfur removal efficiency. Such hydrolysis/hydrogenation pre- and/or post-treatment steps require additional capital and operating costs for the supply of hydrogen, which supply is lacking due to only providing, in accordance with the aforementioned patents (particularly U.S. Pat. Nos. 4,552,746 and 4,857,297) which stipulate that water in the feed gas stream comprise less than 10 vol % water.